Generally, streetlights are lighting devices used for lighting facilities that are installed along the streets for safety and security of road traffic, and thus appropriate kinds of the streetlights are used according to the installation location such as the highways, the major roads, commercial roads, and residential roads of urban district towns.
The lighting devices are equipped with a lamp housing on the inner surface of which a reflector is provided and that is installed on a streetlight pole, and a light source that is mounted in the inside of the lamp housing, respectively, to thus emit light. There are several different types of streetlight poles, which are a highway type of bending the end of each pole and suspending a lamp at the end of the pole, a bracket type of extending a branch horizontally at the end of each pole and suspending a lamp at the branch, and a capital type of suspending a lamp on top of each pole.
Moreover, high-pressure mercury lamps, fluorescent lamps, sodium vapor lamp or normal light bulbs, etc., are used as light sources. These streetlights emit white, yellow or blue light by predetermined color light sources. Of course, the streetlights may be selected according to an electric power efficiency or light intensity of the street lamps or the surrounding atmosphere.
Meanwhile, the streetlights are designed in the form of most efficiently illuminating roads considering a light distribution efficiency of distributing light on the roads at the time of installing the streetlights on the roads. In the case of using bulb type lamps, the light distribution is controlled by adjusting the angle of reflection of the reflector that is provided in the inner surface of the lamp housing so that roads are illuminated with an appropriate light distribution at the time of designing roadway lighting.
Generally, light distribution types that are chiefly used for roadway lighting are classified into first through fifth light distribution types as shown in FIG. 10. Except for some special roads, most of the roads are efficiently illuminated by chiefly using second through fifth light distribution types of light distribution curves.
However, when various lamps used as light sources in the conventional streetlights, that is, the high-pressure mercury lamps, fluorescent lamps, and sodium vapor lamp, etc., are initially manufactured, their brightness and diffusion ranges are determined. Thus, the conventional streetlights are so disadvantageous in the fact that users cannot artificially adjust the brightness and diffusion ranges. In addition, the conventional streetlights are very short in their life cycles and have shortcomings of causing more power consumption.
Considering the above-described problems, lighting devices using LEDs (Light-Emitting Diodes) as light sources are bing recently proposed. According to technological development, LEDs of low power consumption and high brightness light emission have been developed. Such LEDs are being gradually spread in use. The high brightness LEDs include lens portions that can be used by dispersing light emitted from LED chips and by dividing light emitting ranges into for example, 12-degree lens, 25-degree lens, 30-degree lens, 45-degree lens, and so on, when the LED chips are packaged so that the light emitted from the LED chips can illuminate a large area due to a strong straightforwardness of the light emitted from the LED chips.
Nevertheless, the LED lighting devices have the angles of illumination relatively smaller than bulb type light sources whose illumination angles are 360 degrees. Accordingly, the LED lighting devices generally illuminate roads by respectively mounting a number of LED modules on the lower surface of an upper plate of a housing and using a reflector that is provided in the inner side of the lateral surface of the housing.
The lighting devices include the number of the LED modules mounted on the lower surface of the upper plate, in order to secure an angle of view (that is called a cut-off-angle) so that pedestrians or drivers cannot see directly LED modules within a predetermined angle.
However, these high brightness LED streetlights directly interfere with walking or driving in the case that light from the light sources is directly illuminated on the pedestrians' or the drivers' eyes during walking or driving, to thereby cause accidents. Accordingly, it is essential to secure the angle of view or cut-off-angle.
However, the LEDs are semi-permanently long in life cycles when compared to the conventional lamps. Here, brightness of the LEDs is determined by combining a combination of multiple LEDs. Accordingly, in the case that light distribution is formed through a reflector, a light distribution area is small and a brightness degree is low. Further, there is a limitation in realizing a light distribution efficiency of efficiently illuminating roads, that is, the first through fifth light distribution types of the light distribution techniques. Thus, the LED lighting devices provide brightness only depending on the reflector, while excluding the ideal arrangement at the time of manufacturing. As a result, the LED lighting devices may cause inefficient illumination on the roads and thus are not being widely used.
Moreover, it is difficult to efficiently dissipate heat radiating from the number of the LEDs. Thus, because of the heat, a luminous efficiency does not only degrade but also parts of the LEDs are damaged.
In addition, the streetlights using the conventional LED lamps or conventional non-LED lamps have used transparent protective covers in order to protect the lamps that are installed within the housing. However, these protective covers are formed into concave types that mainly protrude to the external side of the housing. As a result, the streetlights are protruded up and down in protruding shape at the state where the housing and the protective cover have been combined, to thereby cause the overall volume to become large and have the difficulty of storage and transportation.